High-strength cold rolled steel sheet and process for producing the same

ABSTRACT

The invention provides a high strength cold rolled steel sheet comprising ferrite phases and second phases, in which the mean grain size of the ferrite phases is 20 μm or less, the volume fraction of the second phase is 0.1% or more to less than 10%, the absolute value |Δr| of in-plane anisotropy of r value is less than 0.15, and the thickness is 0.4 mm or more. The high strength cold rolled steel sheet of the present invention has a tensile strength of 370 to 590 MPa, and has excellent stretchability, dent resistance, surface precision, secondary working embrittlement, anti-aging, and surface appearance, therefore it is suitable for outer panels of automobile.

TECHNICAL FIELD

[0001] The present invention relates to a high strength cold rolledsteel sheet suitable for inner and outer panels of automobile, andparticularly relates to a high strength cold rolled steel sheet havingexcellent stretchability and a tensile strength of 370 to 590 MPa and amethod for manufacturing the same.

BACKGROUND ART

[0002] Recently, weight saving in a steel sheet for automobile has beenpromoted in view of environmental issue, and use of a cold rolled steelsheet having improved strength has been investigated for inner and outerpanels of automobile. The cold rolled steel sheet for inner and outerpanels of automobile is required to have excellent stretchability, dentresistance, surface precision, anti-secondary working embrittlement,anti-aging, and surface appearance, and a high strength cold rolledsteel sheet having such characteristics and a tensile strength of 370 to590 MPa is now strongly desired by automobile manufacturers.

[0003] Before now, for example, JP-A-5-78784 proposes a high strengthcold rolled steel sheet having a tensile strength of 350 to 500 MPa,which comprises a Ti-bearing ultra-low carbon steel added with a largeamount of solid solution hardening elements such as Mn, Cr, Si, or P.

[0004] JP-A-2001-207237 or JP-A-2002-322537 proposes a galvanized steelsheet (dual phase structure steel sheet: DP steel sheet) having atensile strength of less than 500 MPa, which comprises 0.010 to 0.06% C,0.5% or less Si, not less than 0.5% to less than 2.0% Mn, 0.20% or lessP, 0.01% or less S, 0.005 to 0.10% Al, 0.005% or less N, 1.0% or lessCr, wherein (Mn+1.3 Cr) is 1.9 to 2.3%, and consists of ferrite phasesand second phases (low temperature transformation phases) of 20% or lessby area ratio containing martensite phases of 50% or more.

[0005] However, the high strength cold rolled steel sheet described inJP-A-5-78784 has poor anti-aging, bad surface appearance due to a largeamount of Si causing a problem in plating, and poor anti-secondaryworking embrittlement due to a large amount of P.

[0006] On the other hand, the DP steel sheet described inJP-A-2001-207237 or JP-A-2002-322537 does not have such problems sinceit is strengthened by second phases, however, it was found from theinventor's supplementary examination that the steel sheet did not alwayshave sufficient stretchability and therefore it was not alwaysapplicable to outer panels of automobile.

DISCLOSURE OF THE INVENTION

[0007] The present invention aims to provide a high strength cold rolledsteel sheet having a tensile strength of 370 to 590 MPa, which isapplicable to outer panels of automobile such as door or hood producedmainly by stretch forming.

[0008] The object is achieved by a high strength cold rolled steel sheetcomprising ferrite phases and second phases, wherein the mean grain sizeof the ferrite phases is 20 μm or less, the volume fraction of thesecond phases is not less than 0.1% to less than 10%, the absolute valueof in-plane anisotropy of r value |Δr| is less than 0.15, and thethickness is 0.4 mm or more.

[0009] The high strength cold rolled steel sheet, for example, consistsessentially of, by mass %, less than 0.05% C., 2.0% or less Si, 0.6 to3.0% Mn, 0.08% or less P, 0.03% or less S, 0.01 to 0.1% Al, 0.01% orless N, and the balance of Fe.

[0010] The high strength cold rolled steel sheet can be manufacturedusing a method comprising the steps of: cold rolling a hot rolled steelsheet having the above composition and containing second phases of 60%or more by volume fraction at a reduction rate of higher than 60% tolower than 85%, and continuously annealing the cold rolled steel sheetin an α+γ region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIGS. 1A and 1B are schematic views showing microstructures of ahigh strength cold rolled steel sheet of the present invention and aconventional DP steel sheet respectively;

[0012]FIG. 2 is a view illustrating distance 1 among adjacent secondphases M measured along grain boundaries of ferrite phases F;

[0013]FIG. 3 is a relationship between texture and stretchability;

[0014]FIG. 4 is a relationship between reduction rate of cold rollingand Δr after annealing;

[0015]FIG. 5 is a continuous cooling transformation diagram forillustrating structure formation of hot rolled steel sheet according tothe present invention;

[0016]FIG. 6 is a relationship between cooling rate after hot rollingand ═Δr| after annealing;

[0017]FIG. 7 is a relationship between cooling temperature range ΔTafter hot rolling and |Δr| after annealing; and

[0018]FIG. 8 is a relationship between cooling conditions after hotrolling and annealing conditions and Δr.

EMBODIMENTS OF THE INVENTION

[0019] After investigation on a high strength cold rolled steel sheethaving a tensile strength of 370 to 590 MPa suitable for outer panels ofautomobile, it becomes clear that a cold rolled steel sheet havingexcellent stretchability, dent resistance, surface precision,anti-secondary working embrittlement, anti-aging, and surface appearancecan be obtained under the following conditions (1) and (2).

[0020] (1) Second phases comprising mainly martensite phases aredispersed uniformly in fine ferrite phases.

[0021] (2) Absolute value of in-plane anisotropy of r value |Δr| isreduced.

[0022] Hereinafter, the detail will be discussed.

[0023] 1. Microstructure

[0024] As described above, in a steel sheet comprising single ferritephases, harmful elements to outer panels of automobile, such as Si or P,must be added much to strengthen, therefore the object of the presentinvention can not be achieved.

[0025] Thus, the steel sheet should be strengthened by forming dualphase structure comprising ferrite phases and second phases havingmainly martensite phases. However, sufficient stretchability can not beobtained by this structure-strengthening. To obtain sufficientstretchability, the second phases comprising mainly martensite phasesneed to be dispersed uniformly in ferrite phases, which has a mean grainsize of 20 μm or less, at a volume fraction of not less than 0.1% toless than 10%. Such second phases are precipitated at the grainboundaries of the ferrite phases.

[0026] When the mean grain size of ferrite phases exceeds 20 μm, orangepeel is generated at press-forming, resulting in deterioration insurface appearance and deterioration in stretchability. Therefore, themean grain size is made to be 20 μm or less, preferably 15 μm or less,and further preferably 12 μm or less.

[0027] When the volume fraction of second phases comprising mainlymartensite phases is less than 0.1% or 10% or more, sufficientstretchability cannotbe obtained. Therefore, the volume fraction ofsecond phases is made to be not less than 0.1% to less than 10%, andpreferably not less than 0.5% to less than 8%. The second phasescomprising mainly martensite phases may have retained γ phases, bainitephases, pearlite phases, and carbides other than martensite phases in arange of 40% or less, preferably 20% or less, and further preferably 10%or less to attain the object of the present invention.

[0028]FIGS. 1A and 1B are views schematically showing microstructure ofa high strength cold rolled steel sheet of the present invention and aconventional DP steel sheet respectively.

[0029] In the steel sheet of the present invention, fine second phases Mare dispersed uniformly in uniform and fine ferrite phases F and alongthe grain boundaries of the ferrite phases F. On the other hand, in theconventional DP steel sheet, coarse second phases M are dispersednonuniformly in nonuniform and coarse ferrite phases F and along thegrain boundaries of the ferrite phases F.

[0030] Now, as shown in FIG. 2, when the mean grain size of the ferritephases F is assumed to be d (μm), and the mean value of distance 1 amongadjacent second phases M measured along the grain boundaries of theferrite phases F is set to be L (μm), if the following formula (1) issatisfied, YPE1 (yield point elongation) disappears easily, which isadvantageous for reduction of YP (yield point), and makes it possible tofurther improve anti-aging.

L<3.5×d   (1)

[0031] It is more advantageous to satisfy the formula L<3.1×d, and muchmore advantageous to satisfy the formula L<2.4×d.

|Δr|  2.

[0032] In addition to the requirement for microstructure, it isextremely important for improvement of stretchability that the absolutevalue of in-plane anisotropy of r value |Δr| should be less than 0.15.

[0033] Such reduction of the absolute value of in-plane anisotropy of rvalue |Δr| implies that the steel sheet is made to be more isotropic(each r value at 0°, 45°, and 90° to a rolling direction, namely each ofr0, r45, and r90 is equal to 1), and it is considered that the yieldstrength in a biaxial tension region is reduced thereby, therefore thestretchability is improved.

[0034] To further improve isotropy of the steel sheet, it is effectivethat difference between maximum value r_(max) and minimum value r_(min)of the r0, r45, and r90 is 0.25 or less, preferably 0.2 or less, andfurther preferably 0.15 or less. It is further effective that the r90 is1.3 or less, preferably 1.25 or less, and further preferably 1.2 orless.

[0035] It is well known that r value is related to texture of steelsheet.

[0036]FIG. 3 shows a relationship between texture and stretchability,and it is confirmed that if the ratio of an X-ray intensity of{111}<uvw> orientation to that of random texture sample as abscissa is3.5 or more, and the difference between maximum intensity ratio andminimum intensity ratio of the orientation as ordinate is 0.9 or less,or if the steel sheet is more isotropic, excellent stretchability can beobtained. Here, the ratio of the X-ray intensity of {111}<uvw>orientation to that of random texture sample and the difference betweenmaximum intensity ratio and minimum intensity ratio of the orientationare values obtained, for example, by the ODF analysis method using“RINT2000 series application software” (three dimensional pole figuredata processing program). The {111}<uvw> orientation is an orientationexisting on the γ fiber at 54.7° of φ and at 45° of φ2 according toBunge Type output.

[0037] Reduction of the |Δr| is sometimes achieved by performing coldrolling at a reduction rate of higher than 85% as the case of tin plate.However, such a high reduction rate is not preferable for the steelsheet for outer panels of automobile from the view points of coldrolling performance, cost, and quality. Therefore, the present inventionis limited to a high strength cold rolled steel sheet that can beproduced at a reduction rate of lower than 85%, or a high strength coldrolled steel sheet having a thickness of 0.4 mm or more, and thereforethe tin plate is excluded from the present invention.

[0038] 3. Compositions

[0039] The high strength cold rolled steel sheet of the presentinvention, for example, consists essentially of, by mass %, less than0.05% C, 2.0% or less Si, 0.6 to 3.0% Mn, 0.08% or less P, 0.03% or lessS, 0.01 to 0.1% Al, 0.01% or less N, and the balance of Fe.

[0040] C: C is an element required for improving strength of steelsheet, however, when the C content is 0.05% or more, stretchability issignificantly deteriorated, in addition, it is not preferable from theviewpoint of weldability. Accordingly, the C content is made to be lessthan 0.05%. To form second phase having the above volume fraction, the Ccontent is preferably 0.005% or more, and further preferably 0.007% ormore.

[0041] Si: When Si content exceeds 2.0%, surface appearance isdeteriorated, and plating adherence is significantly deteriorated.Accordingly, the Si content is made to be 2.0% or less, preferably 1.0%or less, and further preferably 0.6% or less.

[0042] Mn: Mn is generally effective for preventing cracking of steelslab in hot working by precipitating S in steel sheet as MnS. Moreover,in the present invention, Mn of 0.6% or more needs to be added to stablyform second phases. However, when the Mn content exceeds 3.0%, cost ofslab significantly increases, besides formability of steel sheet isdeteriorated. Accordingly, the Mn content is made to be 0.6 to 3.0%, andpreferably not less than 0.8% to less than 2.5%.

[0043] P: When P content exceeds 0.08%, the anti-secondary workingembrittlement is deteriorated, or alloying property of zinc plating isdeteriorated. Accordingly, the P content is made to be 0.08% or less,and preferably 0.06% or less.

[0044] S: S is a harmful element that deteriorates hot workingperformance of steel and increases sensibility to cracking of steel slabin hot working. Moreover, when the S content exceeds 0.03%, S isprecipitated as fine MnS, resulting in deterioration in formability ofsteel sheet. Accordingly, the S content is made to be 0.03% or less,preferably 0.02% or less, and further preferably 0.015% or less. Fromthe viewpoint of surface appearance, the S content is preferably 0.001%or more, and further preferably 0.002% or more.

[0045] Al: Al contributes to deoxidization of steel, and precipitatesunnecessary solid solution N in steel as AlN. The effect is insufficientwhen Al is less than 0.01%, and saturates when Al exceeds 0.1%.Accordingly, the Al content is made to be 0.01 to 0.1%.

[0046] N: It is not preferable from the viewpoint of anti-aging thatsolid solution N exists in steel, therefore the N content should bepreferably few. When the N content exceeds 0.01%, ductility or toughnessis deteriorated because of existence of excessive nitrides. Accordingly,the N content is made to be 0.01% or less, preferably 0.007% or less,and further preferably 0.005% or less.

[0047] In addition to these elements, at least one element selected from1% or less Cr, 1% or less Mo, 1% or less V, 0.01% or less B, 0.1% orless Ti, and 0.1% or less Nb is effectively added from the followingreasons respectively.

[0048] Cr, Mo: Cr and Mo are effective elements for improvinghardenability and forming second phases stably. Moreover, they are alsoeffective for suppressing softening of heat affected zone (HAZ) formedat welding. To this end, at least one of Cr and Mo of 0.005% or more ispreferably added, and further preferably 0.01% or more. However, whenthe content of each element exceeds 1%, the HAZ is excessively hardened,therefore each of the contents of Cr and Mo is made to be 1% or less,preferably 0.8% or less, and further preferably 0.6% or less.

[0049] V: V is effective for suppressing softening of HAZ formed atwelding. To this end, V is preferably added 0.005% or more, and furtherpreferably 0.007% or more. However, when the V content exceeds 1%, theHAZ is excessively hardened, therefore the V content is made to be 1% orless, preferably 0.5% or less, and further preferably 0.3% or less.

[0050] B: B is an effective element for improving hardenability andforming second phases stably. To this end, B is preferably added 0.0002%or more, and further preferably 0.0003% or more. However, when the Bcontent exceeds 0.01%, the effects are saturated, therefore the Bcontent is made to be 0.01% or less, preferably 0.005% or less, andfurther preferably 0.003% or less.

[0051] Ti, Nb: Ti and Nb act to form nitrides and reduce unnecessarysolid solution N in steel. Improvement of formability of steel sheet canbe expected by reducing solid solution N with Ti or Nb instead of Al. Tothis end, at least one of Ti and Nb is preferably added 0.005% or more,and further preferable 0.008% or less. However, when each of thecontents exceeds 0.1%, the effects are saturated, therefore each of thecontents of Ti and Nb is made to be 0.1% or less, and preferably 0.08%or less. However, when Ti or Nb is added in excess of the amountrequired for reducing solid solution N, carbides of excessive Ti or Nbare formed, which prevents the stable formation of second phases,therefore it is not preferable.

[0052] 4. Manufacturing Conditions

[0053] The high strength cold rolled steel sheet of the presentinvention can be manufactured by cold rolling a hot rolled steel sheethaving the above composition and second phases of 60% or more by volumefraction at a reduction rate of higher than 60% to lower than 85%, andthen continuously annealing the cold rolled steel sheet in an α+γregion. To form second phases more stably after annealing, the annealingtemperature needs to be set in a range from Ac1 transformation point to(Ac1 transformation point+80° C.), and preferably Ac1 transformationpoint to (Ac1 transformation point+50° C.).

[0054] As described above, to realize (1) uniformly dispersing secondphases comprising mainly martensite phases in fine ferrite phases and(2) reducing an absolute value |Δr| of in-plane anisotropy of r value,which are requirements for obtaining a cold rolled steel sheet havingexcellent stretchability, dent resistance, surface precision,anti-secondary working embrittlement, anti-aging, and surface appearancetogether, it is necessary that a hot rolled steel sheet before coldrolling contains second phases of 60% or more by volume fraction,preferably 70% or more, and further preferably 80% or more.

[0055] The mechanism is not completely clear, but considered as follows.

[0056] That is, in the case of the conventional hot rolled steel sheetcomprising ferrite phases and pearlite phases, insufficiently dissolvedcarbides are apt to be present during annealing in an α+γ region, andcoarse γ phases are present ununiformly and sparsely reflecting thedistribution of the pearlite phases in a hot rolled steel sheet. As aresult, a structure comprising coarse ferrite phases and comparativelycoarse second phases that are ununiformly dispersed is formed.

[0057] On the other hand, in the case of a hot rolled steel sheet havingsecond phases of 60% or more by volume fraction as the presentinvention, fine carbides are once dissolved in ferrite phases duringheating process in annealing, and then fine γ phases are generateduniformly and densely from grain boundaries of ferrite phases duringsoaking in an α+γ region. As a result, the ferrite phases become uniformand fine, and the second phases are also dispersed finely and uniformly.In the case of the hot rolled steel sheet containing second phases asthe present invention, a transformation texture is formed unlike thecase of a conventional dual phase steel sheet comprising ferrite phasesand pearlite phases, which gives the apparently same effect as thestrain addition in cold rolling, and the |Δr| can be reduced even at atypical reduction rate of 60 to 85% as described later.

[0058] Here, the second phases in the hot rolled steel sheet areacicular ferrite phases, bainitic ferrite phases, bainite phases,martensite phases, or mixture phases of them.

[0059]FIG. 4 shows a relationship between reduction rate of cold rollingand |Δr| after annealing, wherein such a hot rolled steel sheet havingsecond phases is cold rolled at various reduction rates, and thencontinuously annealed in an α+γ region.

[0060] When the reduction rate of cold rolling is higher than 60% tolower than 85%, the |Δr| of less than 0.15 can be obtained.

[0061] To manufacture a hot rolled steel sheet having second phases of60% or more by volume fraction, it is necessary, for example, that asteel slab having composition within the scope of the present inventionas described above is hot rolled at Ar3 transformation point or higher,and then cooled within two seconds after hot rolling and over atemperature range of 100° C. or more at a cooling rate of 70° C./s orhigher. The rapid cooling allows to suppress formation of ferrite phasesas shown in the continuous cooling transformation diagram of FIG. 5. Thetime to start cooling after hot rolling is preferably within 1.5 sec,and further preferably within 1.2 sec.

[0062]FIG. 6 shows a relationship between cooling rate after hot rollingand |Δr| after annealing. In this case, cooling temperature range ΔT isset to be 150° C.

[0063] When the cooling rate is 70° C./s or higher, the |Δr| is lessthan 0.15. It is more effective that the cooling rate is higher than100° C./s, and preferably higher than 130° C./s.

[0064]FIG. 7 shows a relationship between cooling temperature range ΔTafter hot rolling and |Δr| after annealing. In this case, the coolingrate is set to be 150° C./sec.

[0065] When the cooling temperature range ΔT is 100° C. or more, the|Δr| is less than 0.15. The cooling temperature range ΔT is preferably130° C. or more, and more preferably 160° C. or more.

[0066]FIG. 8 shows a relationship between cooling conditions after hotrolling and annealing conditions and Δr.

[0067] When the continuous annealing is not performed in an α+γ regioneven if the hot rolling conditions as those in the present invention areemployed, or when the continuous annealing is performed in an α+γ regionwithout employing the hot rolling conditions as those in the presentinvention, the Δr value is large. The small Δr can be obtained at anormal reduction rate of cold rolling only when the hot rolling underthe conditions of the present invention is combined with the continuousannealing in an α+γ region. This is the point of the present invention.

[0068] In a manufacturing method according to the present invention, aslab may be hot rolled after being reheated in a furnace, or directlyhot rolled without being reheated. The coiling after hot rolling may beconducted at a temperature at which second phases of 60% or more byvolume fraction can be formed, and under the cooling conditions afterhot rolling of the present invention, normal coiling temperature can beapplicable.

[0069] The continuous annealing can be performed in a present continuousannealing line or a present galvanization line.

[0070] The high strength cold rolled steel sheet of the presentinvention may be subjected to electrolytic galvanization or hot-dipgalvanization. Alloying treatment may be applicable after galvanization.Furthermore, coating may be performed after galvanization.

EXAMPLE

[0071] Steels No. 1 to 15 as shown in Table 1 were melted, and then castinto slabs by continuous casting.

[0072] Steels No. 1 to 11 have composition within the scope of thepresent invention. On the other hand, Steels No. 12 to 15 have any oneof C content, Si content, and Mn content without the scope of thepresent invention. Steels No. 1 to 11 of the present invention have anAr3 transformation point of 820° C. or higher, and an Ac1 transformationpoint and an Ac3 transformation point between 740° C. and 850° C.

[0073] The slabs were reheated to 120° C., hot rolled at finishingtemperatures shown in Table 2, cooled under the conditions of coolingstart time, cooling rate, and cooling temperature range ΔT shown inTable 2, and then coiled at normal coiling temperatures, thereby hotrolled steel sheets were produced. The hot rolled steel sheets werepickled, cold rolled into 0.75 mm in thickness at reduction rates shownin Table 2, and then subjected to continuous annealing in a continuousannealing line (CAL) or a continuous galvanizing line (CGL), therebycold rolled steel sheets No. 1 to 30 having different tensile strengthlevels of 400 MPa or less, more than 400 MPa to not more than 500 MPa,and more than 500 MPa were produced. The annealing was carried out atsoaking temperatures shown in Table 2. Some of the cold rolled steelsheets were subjected to galvanizing in an electrolytic galvanizing line(EGL). These cold rolled steel sheets were finally subjected to temperrolling at a reduction rate of 0.2 to 1.5%.

[0074] Microstructures of the hot rolled steel sheet and the cold rolledsteel sheet were observed using a scanning electron microscope, and thegrain size of ferrite phases, the volume fraction of second phases, themean distance among second phases were obtained through image analysis.JIS No. 5 tensile test piece was used to measure r value and Δr.Furthermore, tensile test was carried out using the JIS 5 tensile testpiece to obtain tensile strength TS and elongation E1 in a directionperpendicular to the rolling direction. To evaluate stretchability, testpiece 200 mm by 200 mm was stretch formed using a hemispherical punch of150 mm in diameter, thereby the limit of stretch height was measured.

[0075] The results are shown in Tables 3-1, 3-2, and 3-3.

[0076] Steels No. 1 to 5, 10, 15, 16, 18, 20, 22, 23, and 25 to 28 inwhich composition, grain size of ferrite phases, volume fraction ofsecond phases, and |Δr| are all within the scope of the presentinvention have a high limit of stretch height and excellentstretchability compared with the comparative examples in which thoseconditions are not within the scope of the present invention, when thecomparison is made in the same strength level.

[0077] Steel No. 7 as a comparative example, which is manufactured underthe same conditions as those of the examples in JP-A-2001-207237 orJP-A-2002-322537, does not have a sufficiently high limit of stretchheight although the volume fraction of second phases is within the scopeof the invention. It seems to be because cooling conditions after hotrolling are without the scope of the present invention, resulting in alarge Δr. TABLE 1 (mass %) Steel No. C Si Mn P S Al N others remarks 10.007 0.02 2.05 0.031 0.016 0.071 0.0022 Cr = 0.62 Steel of theinvention 2 0.012 0.26 1.54 0.026 0.0009 0.015 0.0008 Mo = 0.26, Steelof the Ti = 0.031 invention 3 0.015 0.02 1.50 0.020 0.005 0.050 0.0040Cr = 0.5 Steel of the invention 4 0.018 0.01 1.85 0.005 0.007 0.0280.0016 — Steel of the invention 5 0.023 0.68 2.48 0.035 0.010 0.0490.0019 Cr = 0.15, Steel of the Mo = 0.08, invention V = 0.04 6 0.0280.02 1.65 0.012 0.012 0.039 0.0049 V = 0.35, Steel of the Cr = 0.19invention 7 0.031 0.02 1.20 0.055 0.005 0.045 0.0029 B = 0.0008, Steelof the Nb = 0.033 invention 8 0.035 1.20 1.15 0.068 0.009 0.029 0.0039 —Steel of the invention 9 0.042 0.31 1.90 0.014 0.026 0.044 0.0035 V =0.08 Steel of the invention 10 0.046 0.55 0.88 0.008 0.011 0.048 0.0061Mo = 0.66 Steel of the invention 11 0.049 0.22 1.40 0.025 0.0006 0.0310.0014 B = 0.0038 Steel of the V = 0.05 invention 12 0.061 0.04 1.350.025 0.006 0.049 0.0049 — Comparative steel 13 0.027 2.1 1.54 0.0350.019 0.039 0.0042 — Comparative steel 14 0.046 0.21 3.15 0.011 0.0280.055 0.0034 — Comparative steel 15 0.003 0.03 0.59 0.04 0.009 0.0440.0022 — Comparative steel

[0078] TABLE 2 Cooling Steel Finishing Cooling start temperatureReduction Annealing sheet Steel temperature time Cooling rate range ratetemperature No. No. (° C.) (sec) (° C./sec) ΔT (° C.) (%) (° C.) 1 1 8750.2 250 255 83 775 2 1 880 0.4 195 235 88 770 3 2 880 0.2 245 250 80 7654 2 885 0.5 250 155 80 770 5 2 890 0.3 235 125 80 775 6 2 815 0.8 120175 80 785 7 3 850 2.1 35 205 60 800 8 3 855 0.6 155 255 55 800 9 15 8900.7 165 245 77 825 10 4 870 0.5 205 265 75 770 11 4 865 2.3 210 225 75775 12 4 875 0.8 55 200 75 765 13 4 870 0.9 80 85 75 770 14 4 880 1.8 35230 88 775 15 5 910 0.2 195 230 75 745 16 5 895 0.7 105 220 75 760 17 6890 1.1 165 190 77 730 18 6 885 0.9 175 200 77 780 19 6 895 1.0 180 19577 880 20 7 875 0.3 275 115 71 785 21 13 875 1.3 90 145 73 825 22 8 8700.5 305 135 69 815 23 9 860 1.3 135 225 66 775 24 9 870 1.5 115 210 88780 25 9 865 1.4 120 230 73 765 26 9 885 1.7 130 205 73 840 27 10 8550.3 85 250 71 760 28 11 850 0.4 95 270 63 780 29 14 870 1.6 125 135 75820 30 12 855 0.7 125 185 71 780

[0079] TABLE 3-1 Volume fraction of Volume Mean second fraction distanceL phases Grain size d of among Limiting Steel after hot of ferritesecond second stretching sheet rolling phases phases phases TS EI heightNo. (%) (μm) (%) (μm) 3.5 × d Δr r_(max) − r_(min) r90 (MPa) (%) (mm)remarks 1 93 14.4 0.5 18.5 50.4 0.06 0.16 1.09 374 44.0 60.1 Example ofthe invention 2 83 15.9 0.4 32.1 55.7 −0.01 0.13 1.37 364 39.7 58.0Example of the invention 3 100 10.8 1.4 11.5 37.8 0.04 0.09 1.06 39142.7 59.2 Example of the invention 4 77 11.4 1.2 20.4 39.9 0.11 0.141.08 382 42.9 58.7 Example of the invention 5 62 13.3 0.9 28.2 46.8 0.140.19 1.12 371 43.2 58.2 Example of the invention 6 0 15.9 0.9 56.4 55.70.48 0.63 1.41 377 38.6 54.8 Comparative example 7 0 14.2 3.1 52.2 49.70.34 0.50 1.38 385 37.6 53.4 Comparative example 8 78 13.1 3.3 34.5 45.90.18 0.26 1.21 398 36.1 51.9 Comparative example 9 15 17.3 0 — — 0.310.43 2.05 356 39.9 54.9 Comparative example 10 92 7.9 2.4 9.1 27.7 0.030.05 1.03 442 39.6 56.7 Example of the invention

[0080] TABLE 3-2 Volume fraction of Mean second Volume distance L phasesGrain size d fraction of among Limit of Steel after hot of ferritesecond second stretch sheet rolling phases phases phases TS EI heightNo. (%) (μm) (%) (μm) 3.5 × d Δr r_(max) − r_(min) r90 (MPa) (%) (mm)remarks 11 25 10.4 1.6 25.0 36.4 0.37 0.55 1.37 412 36.5 52.9Comparative example 12 10 9.2 1.3 28.6 32.2 0.54 0.68 1.43 422 35.9 51.7Comparative example 13 0 9.7 1.5 35.1 34.0 0.42 0.58 1.39 417 36.1 51.4Comparative example 14 0 11.3 1.8 40.3 39.6 −0.46 0.49 0.69 409 37.452.3 Comparative example 15 95 6.7 2.6 7.9 23.5 0.06 0.09 1.05 460 38.455.7 Example of the invention 16 68 7.6 1.9 23.5 26.6 0.09 0.12 1.07 44938.6 54.7 Example of the invention 17 87 6.5 0 — — 0.40 0.49 1.24 46133.9 50.4 Comparative example 18 91 6.4 3.4 8.2 22.4 0.06 0.23 1.14 47737.1 55.2 Example of the invention 19 88 8.5 1.1 16.5 29.8 −0.43 0.450.93 465 32.7 49.5 Comparative example 20 69 6.5 4.1 9.3 22.8 0.09 0.221.15 489 36.4 54.1 Example of the invention

[0081] TABLE 3-3 Volume fraction of Mean second Volume distance L phasesGrain size d fraction of among Limit of Steel after hot of ferritesecond second stretch sheet rolling phases phases phases TS EI heightNo. (%) (μm) (%) (μm) 3.5 × d Δr r_(max) − r_(min) r90 (MPa) (%) (mm)remarks 21 45 20.5 0.7 72.5 71.8 0.08 0.43 1.22 452 37.8 50.6Comparative example 22 79 6.2 4.4 14.5 21.7 0.12 0.23 1.21 515 34.8 51.8Example of the invention 23 91 5.9 6.1 6.8 20.7 0.14 0.18 1.14 548 34.251.7 Example of the invention 24 89 8.2 5.9 16.0 28.7 −0.33 0.37 0.79531 30.1 46.5 Comparative example 25 88 6.2 6.2 6.6 21.7 0.00 0.04 1.01545 34.4 51.6 Example of the invention 26 90 7.4 4.9 21.5 25.9 0.09 0.231.25 522 34.3 51.0 Example of the invention 27 98 5.1 7.9 5.6 17.9 0.070.10 1.06 572 33.3 50.2 Example of the invention 28 100 4.1 9.8 5.5 14.40.14 0.18 1.14 590 32.4 49.5 Example of the invention 29 100 5.2 10.85.1 18.2 0.31 0.47 1.38 609 29.2 44 Comparative example 30 91 4.8 14.34.3 16.8 0.48 0.66 1.45 645 28.3 42 Comparative example

1. A high strength cold rolled steel sheet comprising ferrite phases andsecond phases, wherein the mean grain size of the ferrite phases is 20μm or less, the volume fraction of the second phases is not less than0.1% to less than 10%, the absolute value of in-plane anisotropy of rvalue |Δr| is less than 0.15, and the thickness is 0.4 mm or more. 2.The high strength cold rolled steel sheet according to claim 1, whereinthe mean distance L (μm) among the adjacent second phases measured alongthe grain boundaries of the ferrite phases satisfies the followingformula (1) when the mean grain size of the ferrite phases is assumed tobe d (μm): L<3.5×d.   (1)
 3. The high strength cold rolled steel sheetaccording to claim 1, wherein the difference between maximum valuer_(max) and minimum value r_(min) of r values at 0°, 45°, and 90° to arolling direction, or r0, r45, and r90, is 0.25 or less.
 4. The highstrength cold rolled steel sheet according to claim 2, wherein thedifference between maximum value r_(max) and minimum value r_(min) of rvalues at 0°, 45°, and 90° to a rolling direction, or r0, r45, and r90,is 0.25 or less.
 5. The high strength cold rolled steel sheet accordingto claim 1, wherein the r value at 90° to a rolling direction, or r90,is 1.3 or less.
 6. The high strength cold rolled steel sheet accordingto claim 2, wherein the r value at 90° to a rolling direction, or r90,is 1.3 or less.
 7. The high strength cold rolled steel sheet accordingto claim 1 consisting essentially of, by mass %, less than 0.05% C, 2.0%or less Si, 0.6 to 3.0% Mn, 0.08% or less P, 0.03% or less S, 0.01 to0.1% Al, 0.01% or less N, and the balance of Fe.
 8. The high strengthcold rolled steel sheet according to claim 2 consisting essentially of,by mass %, less than 0.05% C, 2.0% or less Si, 0.6 to 3.0% Mn, 0.08% orless P, 0.03% or less S, 0.01 to 0.1% Al, 0.01% or less N, and thebalance of Fe.
 9. The high strength cold. rolled steel sheet accordingto claim 3 consisting essentially of, by mass %, less than 0.05% C, 2.0%or less Si, 0.6 to 3.0% Mn, 0.08% or less P, 0.03% or less S, 0.01 to0.1% Al, 0.01% or less N, and the balance of Fe.
 10. The high strengthcold rolled steel sheet according to claim 4 consisting essentially of,by mass %, less than 0.05% C, 2.0% or less Si, 0.6 to 3.0% Mn, 0.08% orless P, 0.03% or less S, 0.01 to 0.1% Al, 0.01% or less N, and thebalance of Fe.
 11. The high strength cold rolled steel sheet accordingto claim 5 consisting essentially of, by mass %, less than 0.05% C, 2.0%or less Si, 0.6 to 3.0% Mn, 0.08% or less P, 0.03% or less S, 0.01 to0.1% Al, 0.01% or less N, and the balance of Fe.
 12. The high strengthcold rolled steel sheet according to claim 6 consisting essentially of,by mass %, less than 0.05% C, 2.0% or less Si, 0.6 to 3.0% Mn, 0.08% orless P, 0.03% or less S, 0.01 to 0.1% Al, 0.01% or less N, and thebalance of Fe.
 13. The high strength cold rolled steel sheet accordingto claim 7 further containing at least one element selected from 1% orless Cr, 1% or less Mo, 1% or less V, 0.01% or less B, 0.1% or less Ti,and 0.1% or less Nb.
 14. The high strength cold rolled steel sheetaccording to claim 8 further containing at least one element selectedfrom 1% or less Cr, 1% or less Mo, 1% or less V, 0.01% or less B, 0.1%or less Ti, and 0.1% or less Nb.
 15. The high strength cold rolled steelsheet according to claim 9 further containing at least one elementselected from 1% or less Cr, 1% or less Mo, 1% or less V, 0.01% or lessB, 0.1% or less Ti, and 0.1% or less Nb.
 16. The high strength coldrolled steel sheet according to claim 10 further containing at least oneelement selected from 1% or less Cr, 1% or less Mo, 1% or less V, 0.01%or less B, 0.1% or less Ti, and 0.1% or less Nb.
 17. The high strengthcold rolled steel sheet according to claim 11 further containing atleast one element selected from 1% or less Cr, 1% or less Mo, 1% or lessV, 0.01% or less B, 0.1% or less Ti, and 0.1% or less Nb.
 18. The highstrength cold rolled steel sheet according to claim 12 furthercontaining at least one element selected from 1% or less Cr, 1% or lessMo, 1% or less V, 0.01% or less B, 0.1% or less Ti, and 0.1% or less Nb.19. A method for manufacturing a high strength cold rolled steel sheetcomprising the steps of: cold rolling a hot rolled steel sheet havingany one of compositions according to claims 7 to 18 and second phases of60% or more by volume fraction at a reduction rate of higher than 60% tolower than 85%, and continuously annealing the cold rolled steel sheetin an α+γ region.
 20. The method according to claim 19, wherein a hotrolled steel sheet is cooled within two seconds after hot rolled at anAr3 transformation temperature or higher, and over a temperature rangeof 100° C. or more at a cooling rate of 70° C./s or higher.